1. Field
One embodiment of the invention relates to a radio device and an electronic apparatus having the same installed therein, and more particularly to a radio device having multi-resonance type antennas, and an electronic apparatus having the same installed therein.
2. Description of the Related Art
In recent years, various terminals having respective radio devices installed therein have come into wide use. Radio communication antennas installed in these terminals, respectively, are proceeding from whip type antennas which have been widely adopted to built-in antennas. Use of the built-in antenna offers such advantages that the built-in antenna is more readily handled in use and accommodation than the whip type antenna is handled, and the degree of freedom for a chassis design increases, and so forth. In particular, the advantage of being able to further thin the chassis is great.
In the conventional built-in antenna, when the miniaturization of the chassis progresses, so that the built-in antenna is disposed closer to a substrate, an impedance decreases because an antenna element is disposed close to a metallic portion of a peripheral circuit or the like. As a result, impedance mismatching is generated between the antenna and a power supply circuit to cause reduction of performance in some cases.
A folded dipole antenna is known as a technique with which an impedance of an antenna is suitably set so as not to decrease too much. The folded dipole antenna is an antenna in which two or more dipole antennas are disposed in close proximity in parallel to each other, their heads are connected to each other, and one of these dipole antennas is supplied with a power at a central feeding point. This technique is disclosed in a first literature of “Antenna Engineering Handbook”, edited by THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERINGS, Ohom sha Ltd., Japan, October, 1996 (refer to pp. 112 and 113, and FIGS. 4•1 and 4•3). Normally, the dipole antennas are disposed on both sides of the feeding point so as to be symmetrical with respect to the feeding point.
The folded dipole antenna has such a feature that an impedance can be made larger than that of a normal unfolded dipole antenna, and a value of the impedance can be adjusted in accordance with a ratio in line diameter of parallel lines. However, the dipole antenna is unsuitable for an antenna built in a compact apparatus since it is essentially easy to increase in shape, and thus the dipole antenna is folded in more complicated shape.
In addition, the broadening of the band for the antenna is required in accordance with diversification of communication systems and applications of radio devices. In order to comply with this requirement, an antenna needs to be constructed by combining a plurality of antenna elements having different resonance frequencies with one another. The dipole antenna is also disadvantageous in that the antenna elements become largely complicated.
The folded dipole antenna can also be used as a monopole antenna. In this case, one end of one of the antenna elements which are symmetrically constructed in the folded dipole antenna is supplied with a power and the other end of the one thereof is grounded. This monopole antenna is called a folded monopole antenna. The folded monopole antenna has theoretically the characteristics equal to those of the folded dipole antenna, and may adopt a construction which is half in scale that of the folded dipole antenna. Thus, the practical application of the folded monopole antenna to a compact apparatus is examined now. Such a practical application of the folded monopole antenna is disclosed in a second literature of Satoh and Amano: “Two-points short-circuited folded antenna sharing two frequencies”, THE 2004 IEICE GENERAL CONFERENCE B-1-57, MARCH, 2004.
The technique disclosed in the second literature is such that the so-called inverse L type folded monopole antennas each having a relatively low position are combined with each other, and the respective resonance frequencies are made different from each other, thereby realizing multi-resonance promotion.
The prior art disclosed in the second literature has such a feature that it is suitable for thinning the radio device by using the antenna elements each having the relatively low position, and one-side antenna element is short-circuited halfway, thereby obtaining easiness of the impedance adjustment of an opposite-side antenna element in addition to the multi-resonance promotion. However, since it is necessary to combine a plurality of folded antenna elements with each other, there is still room for improvements in order to cope with a restriction in a mounting space accompanying more multi-function promotion for a compact radio device.